Specific cell types and their connectivity are a key determinant in neural function and selectivity. Primary visual cortex (V1) is one of the largest and most complex structures in the brain and several recent technological advances have enabled more detailed probing of cell type specific relationships to connectivity for a range of V1 cell functions, including orientation selectivity, aperture tuning, contrast response functions, an gain control. Nevertheless, technical limitations remain that have largely limited these studies to transgenic mice which lack more complex organization found in higher visual species like cat and monkey. We recently developed viral strategies for accessing specific cell types and specific circuits in non-transgenic species (Liu et al., 2013, Curr Biol) opening the door for unprecedented fine scale study of structure-function relationships in highly visual mammals. For this proposal we will apply these new strategies to enable optogenetic manipulation of specific cell types and circuits in cat V1, including superficial layer inhibitory neurons (Aim 1), long-ran lateral inputs to superficial inhibitory neurons (Aim 2), and layer 5 and 6 subcortical projection neurons that directly (or indirectly) interact with superficial layer neurons (Aim 3). Specific questions that we will address include whether and how orientation selectivity and surround suppression interact and are mediated by each of these circuits. These studies will represent the most direct in vivo assessment of inhibitory neurons and underlying intra- and inter-laminar circuitry of a large, highly visual mammal, advancing our understanding of how basic visual processes arise and depend on complex cortical structure.